Understanding the epigenetic changes that occur in canine cancer can provide insights into the underlying mechanisms of the disease, and help identify new targets for treatment.
Cancer is a complex disease, and despite decades of research, it remains a leading cause of death in dogs over two years old. We know cancer is not a monolithic disease, and that its development is influenced by many external factors, such as genetics and the environment. Mounting evidence suggests that epigenetics also plays a role in cancer development and progression. In this article, we’ll take a closer look at what we know about the relationship between canine cancer and epigenetics.
WHAT IS EPIGENETICS?
Epigenetics studies the changes in gene expression that occur without altering the DNA sequence. This is important to remember, and is different from, for example, a mutation that alters DNA sequence (such as a deletion or frame shift). A variety of environmental factors, such as diet, exercise, stress, and exposure to toxins, can alter gene expression. Epigenetic changes also can be inherited from one generation to the next.
Epigenetic changes can affect gene expression by altering the structure of DNA, modifying histones (proteins that help package DNA into chromatin), and altering the expression of non-coding RNAs (more on this later).
Before we go much further, we probably need a quick refresher on DNA structure and basic molecular biology, in order to understand epigenetics.
- First, we need to remember something called the central dogma of molecular biology: DNA contains the information to code all the proteins in the body, and RNA is the messenger that transmits this information to the ribosomes responsible for making proteins.
- We must also precisely define what we mean by gene expression. This term refers to the process involved in turning the DNA code contained in a particular unit identified as a gene into a “product” — typically a protein but also RNA.
- We should recall from basic biology class that two processes — transcription and translation — are involved in gene expression. Transcription refers to the conversion of DNA code to RNA, and translation is the process occurring in the ribosomes where the information coded by the RNA is used to direct the formation of new proteins.
- Lastly, it’s important to remember that gene expression is regulated — it doesn’t just “happen.” Factors that can affect gene expression include cellular signals, feedback mechanisms, and the environment.
WHAT DOES EPIGENETICS HAVE TO DO WITH CANCER?
In cancer cells, epigenetic changes can alter the expression of genes that regulate cell growth and division, leading to uncontrolled cell growth and the formation of tumors. Epigenetic changes can also affect the ability of cancer cells to invade surrounding tissues and spread to other parts of the body.
One of the most well-studied epigenetic changes in cancer is DNA methylation. As the name implies, DNA methylation refers to the addition of a methyl group to a cytosine nucleotide in a DNA molecule, but also can refer to a loss of a methyl group in other regions. Methylation affects gene expression by altering the accessibility of DNA to transcription factors and other regulatory proteins. In cancer cells, the addition of a methyl group in one location, and loss in another, can result in the silencing of tumor suppressor genes and the activation of oncogenes, promoting tumor growth and progression. As a side note, methylation is also associated with aging and a lot of research is looking at DNA methylation as a marker of unhealthy aging and disease risk.
Histone modifications are another important epigenetic change in cancer. Histones are proteins that help package DNA into chromatin. Modifications to histones, including acetylation and methylation, can once again affect the accessibility of DNA to transcription factors and other regulatory proteins, with the same outcomes noted previously.
Earlier, we talked about the central dogma of DNA to RNA to proteins. However, not all RNA ends up translated into protein; this type of RNA is called non-coding RNA. The term “non-coding” is a bit of a misnomer, since non-coding RNAs, such as microRNAs and long non-coding RNAs, are also important regulators of gene expression. These RNAs can interact with messenger RNAs (mRNAs) to inhibit their translation into proteins or target them for degradation. In cancer cells, non-coding RNAs can be dysregulated, leading to the altered expression of genes that regulate cell growth and division.
WHAT TYPES OF ENVIRONMENTAL FACTORS CAUSE EPIGENETIC CHANGES?
A growing body of research in human medicine looks at how the environment influences epigenetic changes.
The most obvious factor is simply aging. During fetal development, epigenetic changes dictate which cells become nerve cells, muscle cells or heart cells. As we (and our dogs) age, we’re exposed to lots of substances that have the potential to cause changes. In addition, certain epigenetic changes seem to be a part of normal aging.
Several lifestyle factors have been implicated in epigenetic changes and health outcomes (positive and negative), although we need to remember that some are just associated with disease and others have only been demonstrated in vitro. These factors include obesity (negative), exercise (positive), air pollution (negative), smoking (negative), polyphenols in foods (positive), alcohol consumption (negative) and emotional stress (negative).1
An intriguing — and important — fact about epigenetic changes is that they can influence the health of future generations. An often-cited example from human medicine comes from studies of Dutch women who were pregnant during the winter famine of 1944-1945. Over 60 years later, researchers found that these children, now adults, had a higher incidence of certain diseases. Further examination revealed these individuals had higher levels of methylation at some gene loci when compared to siblings not affected by the famine.2-5
There is also evidence that epigenetic changes are reversible. Another example from human medicine comes from studies of smokers. There is plenty of evidence that smoking triggers a variety of epigenetic changes, but when people stop smoking, many of these epigenetic changes reverse themselves.6
Unfortunately, things are a bit murkier when it comes to environmental factors, epigenetic changes, and canine cancer. Exposure to secondhand smoke, air pollution and smog, insecticides, pesticides and herbicides have all been linked to cancer development in dogs, but the exact mechanisms remain unknown.
EPIGENETICS AND CANINE CANCER
Researchers have a growing interest in studying epigenetic changes and their influence on the development of cancer in dogs. Understanding these changes could point to new treatments and possibly even preventive measures for many different types of canine cancer.
DNA methylation patterns are an active area of research. Although a definitive link between methylation patterns and alterations, and cancer development, hasn’t been established, an accumulating body of evidence shows that this process is important.
Several published studies have linked certain hypomethylation patterns to the development of lymphoma, leukemia, high-grade mast cell tumors, and lung cancer. Conversely, hypermethylation of other regions also have been associated with lymphoma, leukemia, and melanoma in dogs.7-17
Histone modifications also have been linked to canine cancer, specifically bladder cancer and osteosarcoma.18-22 Currently, Morris Animal Foundation has one active grant studying a mutation in the SETD2 gene, which encodes a histone methyltransferase. In this instance, methylation is important for normal function, and if mutated, the dysregulation is associated with aggressive osteosarcoma in people. This Foundation-funded study is looking at the same mutation in dogs, and determining if it could ultimately be leveraged as a therapeutic target.
Non-coding RNAs also are implicated in cancer development. Most veterinarians are probably familiar with microRNAs and their association with cancer. Evidence exists for microRNA dysregulation in osteosarcoma, mast cell tumors, lymphoma, mammary cancer, melanoma, and hemangiosarcoma. MicroRNAs in the blood and urine have generated a lot of interest as cancer biomarkers that could be used as screening tools.23-30
NEW EPIGENETIC RESEARCH FUNDED BY MORRIS ANIMAL FOUNDATION
In the last five years, the Foundation has seen a big uptick in the number of proposal submissions focused on epigenetics. Current projects in progress or under consideration include:
- Studying methylation for early detection and recurrence of canine hemangiosarcoma
- Using a DNA methylation clock to study aging in dogs
- Studying the therapeutic utility of SETD2 mutations in canine osteosarcoma
- Studying changes associated with aging of the canine immune system.
We’ve even funded a few cat and wildlife epigenetic studies that could have implications for dogs as well.
The Golden Retriever Lifetime Study was initiated as a cancer risk factor project. A current project involves testing urine and blood samples from dogs diagnosed with lymphoma for evidence of environmental toxin exposure.
Understanding the epigenetic changes that occur in canine cancer can provide valuable insights into the underlying mechanisms of the disease, and help identify new targets for cancer treatment. Epigenetic therapies, such as DNA methyltransferase inhibitors and histone deacetylase inhibitors, have shown promise in human cancer treatment and may also be effective in the treatment of canine cancer.
AUTHOR PROFILE
Dr. Kelly Diehl received her DVM from the University of Tennessee and started her practice career in an emergency clinic in New Jersey. She then completed an internship at the prestigious Animal Medical Center in New York City, after which she moved west, completing a residency in small animal medicine at Colorado State University. Dr. Diehl joined the staff of the Veterinary Referral Center of Colorado as the co-owner of the internal medicine section. After 14 years, she left private practice to pursue a career in medical communication and joined the Morris Animal Foundation team in 2013. Dr. Diehl is a board-certified small animal internal medicine specialist and a Certified Veterinary Journalist.
